SIM card swap between iPhone 5 and 5s?

Hello, Japan. My wife will be upgrading her flip-phone to an iPhone 5s today, but she doesn't really care for the bells and whistles of the new iPhone, so I am hoping to switch out her phone for my iPhone 5. Softbank says this will cost us a fee to reprogram the SIMs as well as loss of discounts at time of signing.

Is it possible to just swap the SIM cards at home and reboot them up? Wouldn't this give the other phone the number tied to the SIM card, then just a matter of syncing with my iTunes library and her library on that specific phone? We are both on Softbank.

Thanks in advance!

Yes, it is possible to just swap the sim cards and resync with itunes without telling softbank. They will know that the phones and sim cards don't match though, and theoretically you could be charged for exact data rather than the extremely reduced pakehodai rates among other things. It's highly unlikely, but do so at your own risk.